RICHMOND, Va. — Former Gov. Bob McDonnell of Virginia and his wife, Maureen, on trial for conspiring to use his office for personal enrichment, outlined an unexpected defense on Tuesday: Their marriage was so broken that they did not communicate enough to conspire about anything.

In opening arguments in the couple’s corruption trial in federal court here, their lawyers made clear that they planned to rely on the sordid details of their unhappy union as the basis of their legal defense. It was the first time the McDonnells’ version of events had been heard in a widely publicized case that for months has been characterized by the lengthy indictment against them, which charges the couple with accepting more than $165,000 in cash and luxury gifts from a Virginia businessman.

Ms. McDonnell, her lawyer said, had a “crush” on the businessman, Jonnie R. Williams Sr., the prosecutors’ star witness, who the government said would detail the designer clothing, vacations, golf rounds and cash he provided in exchange for the governor’s help in promoting his company, which made a dietary supplement.

Mr. Williams was a frequent visitor to the Executive Mansion, where he and Ms. McDonnell would meet privately. He was known as “Maureen’s favorite playmate,” the lawyer, William A. Burck, told jurors. “Maureen McDonnell and Jonnie Williams had a relationship some would consider improper for two people not married.”

Over two years, Ms. McDonnell and Mr. Williams exchanged 1,200 phone calls and texts.

The government said that for Mr. Williams, it was about making money. Jessica Aber, the assistant United States attorney trying the case, said: “For Mr. Williams, this was a business transaction. He was paying for help with his company.”

According to both sides, Mr. Williams gained access to the governor by befriending the state’s first lady, spending about $19,000 on a New York City shopping spree for designer clothes and accessories, and buying a $6,500 Rolex watch for her to present to her husband.

The motives of Mr. Williams, a serial entrepreneur known variously as a super salesman and someone whose businesses repeatedly ran afoul of government regulators, are likely to be front and center in the trial.

If convicted on all counts, the McDonnells face more than 20 years in prison. Mr. McDonnell, limited to one term under state law, left office in January.

Defense lawyers said Mr. Williams repeatedly changed his account of his relationship with the McDonnells — first denying he sought anything in return for his generosity and later saying they had “an arrangement” — after the government promised him immunity in a separate case involving his company, formerly named Star Scientific. Mr. Williams was being investigated for possible securities and tax violations over a $10 million transaction at Star Scientific, Mr. Burck said.

“Why did his story change so much?” Mr. Burck said. “Because it gets him a get-out-of- jail card worth $10 million.”

Prosecutors do not portray Mr. Williams as an angel. Ms. Aber described him dismissively as “a vitamin salesman.” But she alleged a quid pro quo between Mr. Williams’s gifts to the McDonnells and the governor’s actions on his behalf. She presented a text message from Mr. McDonnell to Mr. Williams in May 2012 reading, “Per voice mail, would like to see if you could extend another 20k loan for this year.”

Mr. Williams’s response: “Done.”

Another time, after thanking Mr. Williams for funds to help cover expenses for some real estate investments, Mr. McDonnell sent an email to his policy director just six minutes later telling him to meet to discuss how to advance Mr. Williams’s products.

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Jonnie R. Williams Sr., a government witness, and Maureen McDonnell at a reception at the Executive Mansion in 2011CreditMichaele White/Office of the Governor of Virginia, via Associated Press

The aide’s reply: “We need to be careful with this issue.”

Mr. McDonnell’s defense argued such interventions were not improper; they are what all elected officials do to promote state businesses. “To criminalize this bedrock principle,” John Brownlee, one of the former governor’s lawyers, said, “would make felons of virtually every person who’s held public office.”

He made clear he would make Mr. McDonnell’s character central to his defense, calling him “a man of integrity” who was an Army veteran and a lifelong public servant.

He promised jurors the former governor would “take that chair right there” and detail his failed marriage, as a way of showing how his wife had become vulnerable to the ingratiating Mr. Williams.

“She was angry for not having enough money, she was angry at him for not spending enough time at home with her, and she hated him for not being available,” Mr. Brownlee said of Ms. McDonnell, as the former first couple sat at the same table with their separate legal teams.

“Bob and Maureen’s communication broke down almost entirely,” Mr. Brownlee said, with Mr. McDonnell looking on, his eyes grim and his expression wan.

“This tore the marriage apart, and it created a rift so wide an outsider — in this case a man — could invade and poison the marriage,” Mr. Brownlee said.

He added that at a time when prosecutors say the McDonnells were working together to help Mr. Williams, in September 2011, the governor wrote a long email to his wife “begging her to help save the marriage.”

Mr. McDonnell planned to read the deeply personal email in court, his lawyer said. “Bob has agreed to open up his marriage and his life to you, so you can see for yourselves that the government’s allegations he was conspiring with his wife are simply not true,” Mr. Brownlee said.

In exchange for Mr. Williams’s largess, prosecutors charge that Mr. McDonnell arranged for him to pitch his product to the state health secretary; that the McDonnells were hosts of a product promotion at the Executive Mansion for Mr. Williams; and that the first couple tried to help Mr. Williams set up scientific studies of a Star Scientific dietary supplement, Anatabloc, at Virginia’s public universities, to increase its credibility with investors and consumers.

None of these efforts resulted in a measurable gain for Mr. Williams’s product, however. State health officials expressed skepticism about Anatabloc, which is derived from tobacco. Prosecutors said that was beside the point. “Mr. Williams did not have to get a dime of state cash,” Ms. Aber said. Corruption laws were broken simply if “Mr. McDonnell agreed to take official actions on behalf of Mr. Williams” in exchange for his gifts, she said.

After opening statements, the prosecution put the McDonnells’ younger daughter, Cailin McDonnell Young, one of their five grown children, on the stand. Her wedding in June 2011 at the Executive Mansion was paid for in part with a $15,000 gift from Mr. Williams, after Ms. McDonnell had told him that the first couple could not afford the wedding, according to the government.

Ms. Young was shown a wedding photo of two of her brothers and her mother, who prosecutors say was wearing a dress bought by Mr. Williams. Ms. Young began crying. As the prosecutor asked about a family vacation the next month at a multimillion-dollar lake house belonging to Mr. Williams, which included her father’s use of Mr. Williams’s Ferrari, Ms. Young broke down again, and the judge ordered a break.

The trial is expected to last four to five weeks.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Ex-First Couple Has a Defense: State of Union. Order Reprints | Today’s Paper | Subscribe